The letter requests access to all communications between the US Department of Defense and Trump Turnberry, as well as any related financial records.

According to various reports in the US media, the department has not yet complied with the demands.

It has also not commented directly, and neither has the Trump Organisation.

What does the letter say?

The letter – signed by the Democratic committee chairman Elijah Cummings – was addressed to then-acting Secretary of Defence Patrick Shanahan.

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Media captionDonald Trump on the Turnberry golf course in 2014

Citing Defence Logistics Agency (DLA) records, it said the US military had made 629 fuel purchase orders at the airport, totalling $11m (£9m), since October 2017.

It also alleges that certain military personnel have been offered “cut-price rooms” and free rounds of golf at the Trump Turnberry resort.

It continued: “Given the president’s continued financial stake in his Scotland golf courses, these reports raise questions about the president’s potential receipt of US or foreign government emoluments in violation of the US Constitution and raise other serious conflict of interest concerns.”

Who owns the airport?

Prestwick airport, south of Glasgow, is approximately 20 miles (30km) north of Trump Turnberry.

The Scottish government bought it for £1 in 2013, when it was facing closure.